It is well known that integrated circuit fabrication on semiconductor wafers requires the formation of precisely controlled apertures, such as contact openings or "vias," that are subsequently filled and interconnected to create components and very large scale integration (VLSI) or ultra large scale integration (ULSI) circuits. Equally well known is that the patterns defining such openings are typically created by optical lithographic processes that require precise alignment with prior levels to accurately contact the active devices located in those prior levels.
It is also well known that alignment of one pattern layer to previous layers is done with the assistance of special alignment patterns (i.e., alignment marks) designed onto each mask level. When these special patterns are aligned, it is accepted that the remainder of the circuit patterns are also correctly aligned. The adjustment of the image of the mask being exposed to the previously produced patterns was originally performed by human operators, who compared the image locations under a microscope and adjusted the position of the mask to bring it into alignment with the wafer patterns. Decreasing feature sizes and the increasing number of alignments per wafer, however, have promoted the development of automatic alignment systems for use with projection aligners.
One type of conventional automatic alignment procedure involves the use of alignment marks consisting of two rectangular patterns, which are parallel to the directions of the stage motion. In most instances, these alignment marks are formed in the kerf or "street" that separates the individual integrated circuits formed on the semiconductor wafer. Two corresponding rectangular patterns are located on the reticle, and their image is projected onto the wafer. The superimposed alignment target and the reticle image are reflected back into the main optical element of the aligner, and then into an on-axis microscope. The image from the microscope is focused onto the face of a TV camera and is subsequently digitized into a form that can be analyzed by a computer. When alignment is achieved, a signal is obtained.
Recently, manufacturers have encountered problems with alignment marks during the manufacturing process. During conventional metal chemical metal polishing (CMP) of metal stacks, an oxidant is used to convert the top metal to metal oxides. These metal oxides are subsequently abraded in-situ with some harder metal oxide abrasives. The oxidation of the top metal is invariably an exothermic process, which leads to enhanced process temperatures. These increases temperatures increase significantly when a large number of wafers is polished simultaneously (i.e., larger batch size).
During metal CMP, areas dense in features (i.e., alignment marks) tend to oxidize at a faster rate than areas with sparse distributions. This uncontrollable oxidation of the metals forming the alignment marks is commonly referred to as oxide erosion. Additionally, manufacturers have observed that oxide erosion in dense arrays increases dramatically as batch sizes are increased. In such instances, the alignment marks may be either completely destroyed or severely damaged by this erosion. Whether damaged or destroyed, the alignment marks are useless once altered by the erosion since they are no longer optically aligned parallel to the directions of the stage motion.
Accordingly, what is needed in the art is a method of controlling and minimizing the oxidation and resulting oxide erosion that occurs during a CMP process.